Anticoagulation in Cardiopulmonary Bypass

Prolonging the activated clotting time (ACT) 2 to 3 times normal is said to provide a “safe” level of anticoagulation during cardiopulmonary bypass. To test this level of anticoagulation 9 monkeys were anticoagulated with heparin at the start of cardiopulmonary bypass so that ACT’s ranged from 201 sec to > 1000 sec (normal 91 sec). ACT, platelet count (P), fibrinogen (F), and fibrin monomer (FM) were measured at 10, 30, 60, 90, and 120 minutes during bypass. Antithrombin III (AT3) was measured before and after bypass. Six monkeys developed increased FM indicating active coagulation beginning from 10 to 60 minutes on bypass. ACT’s were > 200 sec in all animals at the time of FM detection. P fell below 100,000/mm3 in the 6 animals with elevated FM, but remained above 100,000/mm3 in the other 3 animals. The mean value of AT3 (69%) decreased to 24.4% after bypass in the 6 animals with elevated FM, but was 61% after bypass in the others. Scanning electron microscopy of the oxygenator membranes showed significant amounts of fibrin on the membranes used in monkeys who developed increased FM levels, but not on those with normal FM concentrations. F decreased from 167 mgm/dl to 80.5 mgm/dl in monkeys with elevated FM and to 117 mgm/dl in those with normal FM concentrations. Excessive bleeding did not occur in the animals without increased FM although ACT’s were in excess of 1000 sec. Subsequently three human subjects on cardiopulmonary bypass whose ACT’s were maintained above 400 sec have not shown increased FM levels. The results suggest that prolonging the ACT more than 2 - 3 times normal is required to prevent activation of clotting during cardiopulmonary bypass.